Method for preparing an image element for making an improved printing plate according to the silver salt diffusion transfer process

ABSTRACT

According to the present invention there is provided a method for preparing an imaging element comprising the steps of coating in the order given on a grained and anodized side of an aluminum support (i) an image receiving layer containing physical development nuclei, (ii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer and (iii) optionally an antistress layer being in water permeable relationship with said image receiving layer comprising unhardened gelatin, characterized in that said aluminum support when being coated with said photosensitive layer is in contact with a guiding roller, heated at a temperature between 35 and 70° C.

This application claims benefit of Provisional Appl. Ser. No. 60/081,900filed Apr. 16, 1998.

FIELD OF THE INVENTION

The present invention relates to a method for preparing an aluminum foilsuitable for use as a support for an imaging element for making aprinting plate according to the silver salt diffusion transfer process.

More specifically the invention is related to a method for preparing analuminum foil suitable for use as a support for an imaging element formaking a printing plate according to the silver salt diffusion transferprocess which has improved coating properties.

BACKGROUND OF THE INVENTION

The principles of the silver complex diffusion transfer reversalprocess, hereinafter called DTR-process, have been described e.g. inU.S. Pat. No. 2,352,014 and in the book "Photographic Silver HalideDiffusion Processes" by Andre Rott and Edith Weyde--The FocalPress--London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wiseexposed photographic silver halide emulsion layer material istransformed with a so-called silver halide solvent into soluble silvercomplex compounds which are allowed to diffuse into an image receivingelement and are reduced therein with a developing agent, generally inthe presence of physical development nuclei, to form a silver imagehaving reversed image density values ("DTR-image") with respect to theblack silver image obtained in the exposed areas of the photographicmaterial.

A DTR-image bearing material can be used as a planographic printingplate wherein the DTR-silver image areas form the water-repellentink-receptive areas on a water-receptive ink-repellent background.

The DTR-image can be formed in the image receiving layer of a sheet orweb material which is a separate element with respect to thephotographic silver halide emulsion material (a so-called two-sheet DTRelement) or in the image receiving layer of a so-calledsingle-support-element, also called mono-sheet element, which containsat least one photographic silver halide emulsion layer integral with animage receiving layer in water-permeable relationship therewith. It isthe latter mono-sheet version which is preferred for the preparation ofoffset printing plates by the DTR method.

Two main types of mono-sheet DTR materials that are distinct because oftheir different layer arrangement and processing are known. The firsttype of mono-sheet DTR material comprises on a support, generally paperor a resin support such as polyester, in the order given a silver halideemulsion layer and an image receiving layer containing physicaldevelopment nuclei as a surface layer. After information-wise exposureand development according to the DTR process a silver image is formed inthe surface layer. Since the underlying layers are hydrophilic in natureand the silver image formed on the surface is hydrophobic or can berenderred hydrophobic the thus obtained plate can be used withoutfurther processing. These type of printing plates have a low printingendurance typically around 10000 copies.

On the other hand mono-sheet DTR materials are known that comprise ahydrophilic support provided with an image receiving layer containingphysical development nuclei and on top thereof a silver halide emulsionlayer. After information-wise exposure and development according to theDTR-process a silver image is formed in the image receiving layer. Inorder to obtain a lithographic printing plate it will then be necessaryto remove the now useless silver halide emulsion layer to expose thesilver image formed in the image receiving layer. Said removal isgenerally carried out by rinsing the element with cold or warm water.This type of printing plate is disclosed in e.g. EP-A- 278 766, EP-A-483 415 and EP-A- 410 500.

As a preferred support for the latter type of printing plates aroughened and anodized aluminum foil is used and high printingendurances can in principal be obtained. Such type of supports are wellknown for preparing printing plates using an imaging element having as alight sensitive coating photopolymers (hereinafter called PS-plates)instead of silver halide and are disclosed in e.g. DE-P- 3 717 757,EP-A- 167 751, DE-P- 3 036 174, U.S. Pat. Nos. 4,336,113, 4,374,710,3,980,539, 3,072,546, 3,073,765, 3,085,950, 3,935,080 and 4,052,275.

However the requirements imposed on the aluminum foils for use assupports for PS-plates are different from the requirements imposed onthe aluminum foils for use in the silver salt diffusion transferprocess. Indeed, commonly employed aluminum foils as supports forPS-plates are not suited for preparing printing plates according to thesilver salt diffusion transfer process.

In order to obtain printing plates according to the DTR process havinggood printing properties i.e. good ink acceptance in the image areas, noink acceptance in the non-image areas called staining or toning and highprinting endurances it is required that the adhesion of the imagereceiving layer containing the physical development nuclei and the otherlayers to the aluminum foil is overall firm. Recently is has beenobserved that during the coating with the different layers tinyair-bubbles are formed on the aluminum support. This results afterdevelopment in white spots in the image areas (the exposed areas) and indark spots in the form of a horseshoe (furtheron called horsheshoes) inthe non-imaging areas. Plates with these defaults, wich have a diameterof 0.1 to 1 mm yield print of low quality. So, a solution for saiddefaults is required.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a method forpreparing an aluminum based mono-sheet DTR material for preparing alithographic printing plate having good printing properties.

It is a further object of the present invention to provide a method forpreparing an aluminum based monosheet DTR material which shows afterimage-wise exposure and development no white spots on the image areasand no horsheshoes in the non-image areas

Further objects of the present invention will be clear from thedescription hereinafter.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method forpreparing an imaging element comprising the steps of coating in theorder given on a grained and anodized side of an aluminum support (i) animage receiving layer containing physical development nuclei, (ii) aphotosensitive layer containing a silver halide emulsion being in waterpermeable relationship with said image receiving layer and (iii)optionally an antistress layer being in water permeable relationshipwith said image receiving layer comprising unhardened gelatin,characterized in that said aluminum support when being coated with saidphotosensitive layer is in contact with a guiding roller, heated at atemperature between 35 and 70° C.

DETAILED DESCRIPTION OF THE INVENTION

The aluminum foil is guided over a roller, preferably a bare roller,more preferably a bare metallic roller when said foil is coated by aphotosensitive layer. According to the invention said roller is heatedat a temperature between 35 and 70° C., more preferably at a temperaturebetween 40 and 60° C.

The present invention provides an imaging element comprising in theorder given on a grained and anodized side of an aluminum support (i) animage receiving layer containing physical development nuclei, (ii) aphotosensitive layer containing a silver halide emulsion being in waterpermeable relationship with said image receiving layer and (iii)optional an antistress layer being in water permeable relationship withsaid image receiving layer comprising unhardened gelatin.

The aluminum support of the imaging element for use in accordance withthe present invention can be made of pure aluminum or of an aluminumalloy, the aluminum content of which is at least 95%. The thickness ofthe support usually ranges from about 0.13 to about 0.50 mm.

Preferably the aluminum foil has a roughness with a CLA value between0.2 and 1.5 μm and an anodization layer with a thickness between 0.4 and2.0 μm.

Graining and anodization of the foil are necessary to obtain alithographic printing plate that allows to produce high-quality printsin accordance with the present invention. Sealing is not necessary butmay still improve the printing results. Preferably the aluminum supporthas 500 or more pits per 100 mm2, diameters of the pits being 0.03 to0.30 mm and an average diameter of the pits being 0.05 to 0.20 mm.Preferably the aluminum foil has a roughness with a CLA value between0.2 and 1.5 mm, an anodization layer with a thickness between 0.4 and2.0 mm and is posttreated with an aqueous bicarbonate solution.

According to the present invention the roughening of the aluminum foilcan be performed according to the methods well known in the prior art.The surface of the aluminum substrate can be roughened either bymechanical, chemical or electrochemical graining or by a combination ofthese to obtain a satisfactory adhesiveness of a silver halide emulsionlayer to the aluminum support and to provide a good water retentionproperty to the areas that will form the non-printing areas on the platesurface.

The electrochemical graining process is preferred because it can form auniform surface roughness having a large average surface area with avery fine and even grain which is commonly desired when used forlithographic printing plates.

The roughening is preferably preceded by a degreasing treatment mainlyfor removing greasy substances from the surface of the aluminum foil.

Therefore the aluminum foil may be subjected to a degreasing treatmentwith a surfactant and/or an aqueous alkaline solution.

Preferably roughening is followed by a chemical etching step using anaqueous solution containing an acid. The chemical etching is preferablycarried out at a temperature of at least 30° C. more preferably at least40° C. and most preferably at least 50° C.

After roughening and optional chemical etching the aluminum foil isanodized which may be carried out as follows.

An electric current is passed through the grained aluminum foil immersedas an anode in a solution containing an acid. An electrolyteconcentration from 1 to 70% by weight can be used within a temperaturerange from 0-70° C. The anodic current density may vary from 1-50 A/dm²and a voltage within the range 1-100 V to obtain an anodized film weightof 1-8 g/m² Al₂ O₃.H₂ O. The anodized aluminum foil may subsequently berinsed with demineralised water within a temperature range of 10-80° C.

After the anodizing step sealing may be applied to the anodic surface.Sealing of the pores of the aluminum oxide layer formed by anodizationis a technique known to those skilled in the art of aluminumanodization. This technique has been described in e.g. the"Belgisch-Nederlands tijdschrift voor Oppervlaktetechnieken vanmaterialen", 24ste jaargang/januari 1980, under the title"Sealing-kwaliteit en sealing-controle van geanodiseerd Aluminium".Different types of sealing of the porous anodized aluminum surfaceexist.

Preferably, said sealing is performed by treating a grained and anodizedaluminum support with an aqueous solution containing a bicarbonate asdisclosed in EP-A- 567 178, which therefor is incorporated herein byreference or by treating with a solution of polyvinyl phosphonic acid.

Preferably each of the above described steps is separated by a rinsingstep to avoid contamination of the liquid used in a particular step withthat of the preceding step.

To promote the image sharpness and, as a consequence thereof, thesharpness of the final printed copy, the anodization layer may becoloured in the mass with an antihalation dye or pigment e.g. asdescribed in JA-Pu- 58/14 797.

Subsequent to the preparation of the aluminum foil as described abovethe aluminum foil may be immediately coated with a solution containingthe physical development nuclei or may be coated with said solution at alater stage.

The image receiving layer containing physical development nuclei ispreferably free of hydrophilic binder but may comprise small amounts upto 80% by weight of the total weight of said layer of a hydrophiliccolloid e.g. polyvinyl alcohol to improve the hydrophilicity of thesurface.

Preferred development nuclei for use in accordance with the presentinvention are sulphides of heavy metals e.g. sulphides of antimony,bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, andzinc. Especially suitable development nuclei in connection with thepresent invention are palladium sulphide nuclei. Other suitabledevelopment nuclei are salts such as e.g. selenides, polyselenides,polysulphides, mercaptans, and tin (II) halides. Heavy metals,preferably silver, gold, platinum, palladium, and mercury can be used incolloidal form.

The aluminum support according to the present invention is especiallysuited for preparing a mono-sheet DTR material. According to the methodof the present invention for obtaining a mono-sheet DTR material analuminum foil prepared as described above and provided with an imagereceiving layer is provided with a photosensitive layer in waterpermeable contact with said image receiving layer.

Layers being in waterpermeable contact with each other are layers thatare contiguous to each other or only separated from each other by (a)waterpermeable layer(s). The nature of a waterpermeable layer is suchthat it does not substantially inhibit or restrain the diffusion ofwater or of compounds contained in an aqueous solution e.g. developingagents or the complexed silver.

The photosensitive layer used according to the present invention may beany layer comprising a hydrophilic colloid binder and at least onesilver halide emulsion, at least one of the silver halide emulsionsbeing photosensitive.

The photographic silver halide emulsion(s) used in accordance with thepresent invention can be prepared from soluble silver salts and solublehalides according to different methods as described e.g. by P. Glafkidesin "Chimie et Physique Photographique", Paul Montel, Paris (1967), by G.F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London(1966), and by V. L. Zelikman et al in "Making and Coating PhotographicEmulsion", The Focal Press, London (1966).

For use according to the present invention the silver halide emulsion oremulsions preferably consist principally of silver chloride while afraction of silver bromide may be present ranging from 1 mole % to 40mole %. Most preferably a silver halide emulsion containing at least 70mole% of silver chloride is used.

The average size of the silver halide grains may range from 0.10 to 0.70μm , preferably from 0.25 to 0.45 μm.

Preferably during or after the precipitation stage iridium and/orrhodium containing compounds or a mixture of both are added. Theconcentration of these added compounds ranges from 10⁻⁸ to 10⁻³ mole permole of AgNO₃, preferably between 10⁻⁷ and 10⁻⁵ mole per mole of AgNO₃.

The emulsions can be chemically sensitized e.g. by addingsulphur-containing compounds during the chemical ripening stage e.g.allyl isothiocyanate, allyl thiourea, and sodium thiosulphate. Alsoreducing agents e.g. the tin compounds described in BE-P- 493 464 and568 687, and polyamines such as diethylene triamine or derivatives ofaminomethane-sulphonic acid can be used as chemical sensitizers. Othersuitable chemical sensitizers are noble metals and noble metal compoundssuch as gold, platinum, palladium, iridium, ruthenium and rhodium. Thismethod of chemical sensitization has been described in the article of R.KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

Apart from negative-working silver halide emulsions that are preferredfor their high photosensitivity, use can be made also of direct-positivesilver halide emulsions that produce a positive silver image in theemulsion layer(s) and a negative image on the image-receiving layer.

Suitable direct positive silver halide emulsions for use in accordancewith the present invention are silver halide emulsions that have beenpreviously fogged or that mainly form an internal latent image.

The silver halide emulsions of the DTR-element can be spectrallysensitized according to the spectral emission of the exposure source forwhich the DTR element is designed.

Suitable sensitizing dyes for the visible spectral region includemethine dyes such as those described by F. M. Hamer in "The Cyanine Dyesand Related Compounds", 1964, John Wiley & Sons. Dyes that can be usedfor this purpose include cyanine dyes, merocyanine dyes, complex cyaninedyes, complex merocyanine dyes, homopolar cyanine dyes, hemicyaninedyes, styryl dyes and hemioxonol dyes. Particularly valuable dyes arethose belonging to the cyanine dyes, merocyanine dyes, complexmerocyanine dyes.

In the case of a conventional light source, e.g. tungsten light, a greensensitizing dye is needed. In case of exposure by an argon ion laser ablue sensizing dye is incorporated. In case of exposure by a red lightemitting source, e.g. a LED or a HeNe laser a red sensitizing dye isused. In case of exposure by a semiconductor laser special spectralsensitizing dyes suited for the near infra-red are required. Suitableinfra-red sensitizing dyes are disclosed in i.a. U.S. Pat. Nos.2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921,3,582,344, 3,623,881 and 3,695,888.

A preferred blue sensitizing dye, green sensitizing dye, red sensitizingdye and infra-red sensitizing dye in connection with the presentinvention are described in EP-A- 554 585.

To enhance the sensitivity in the red or near infra-red region use canbe made of so-called supersensitizers in combination with red orinfra-red sensitizing dyes. Suitable supersensitizers are described inResearch Disclosure Vol. 289, May 1988, item 28952. The spectralsensitizers can be added to the photographic emulsions in the form of anaqueous solution, a solution in an organic solvent or in the form of adispersion.

The silver halide emulsions may contain the usual emulsion stabilizers.Suitable emulsion stabilizers are azaindenes, preferably tetra- orpenta-azaindenes, especially those substituted with hydroxy or aminogroups. Compounds of this kind have been described by BIRR in Z. Wiss.Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitable emulsionstabilizers are i.a. heterocyclic mercapto compounds.

As binder in the silver halide emulsion layer(s) in connection with thepresent invention a hydrophilic colloid may be used, usually a protein,preferably gelatin. Gelatin can, however, be replaced in part orintegrally by synthetic, semi-synthetic, or natural polymers. Preferablythe silver halide emulsion layer contains at least one gelatin specieswhereof a 10% by weight aqueous solution at 36° C. and pH 6 has aviscosity lower than 20 mPa·s at a shearing rate of 1000 s⁻¹ combinedwith a gelatin of a higher viscosity. The weight ratio of said lowviscosity gelatin versus the gelatin of a higher viscosity is preferably>0.5.

Preferably the gelatin layer(s) is(are) unhardened. Unhardened meansthat when such gelatin layer is coated on a subbed polyethyleneterephtalate film base at a dry thickness of 1.2 g/m2, dried for 3 daysat 57° C. and 35% R.H. and dipped in water of 30° C., said gelatin layeris dissolved for more than 95% by weight within 5 minutes.

The silver halide emulsions may contain pH controlling ingredients.Preferably at least one gelatin containing layer is coated at a pH valuenot below the iso-electric point of the gelatin to avoid interactionsbetween said gelatin containing coated layer and the hereafter mentionedintermediate layers. More preferably the gelatin layer contiguous tosaid intermediate layer is coated at a pH value not below theiso-electric point of the gelatin. Most preferably all the gelatincontaining layers are coated at a pH value not below the iso-electricpoint of their gelatin. Other ingredients such as antifogging agents,development accelerators, wetting agents, and hardening agents forgelatin may be present.

The silver halide emulsion layer may comprise light-screening dyes thatabsorb scattering light and thus promote the image sharpness. Suitablelight-absorbing dyes are described in i.a. U.S. Pat. Nos. 4,092,168,4,311,787 and DE-P- 2 453 217.

More details about the composition, preparation and coating of silverhalide emulsions suitable for use in accordance with the presentinvention can be found in e.g. Product Licensing Index, Vol. 92,December 1971, publication 9232, p. 107-109.

Preferably, the imaging element also comprises an intermediate layerbetween the image receiving layer and the photosensitive layer(packet)to facilate the removal of said layer(packet) thereby uncovering thesilver image formed in the image receiving layer by processing theimaging element.

In one embodiment, the intermediate layer is a water-swellableintermediate layer coated at a ratio of 0.01 to 2.0 g/m2 and comprisingat least one non-proteinic hydrophilic film-forming polymer e.g.polyvinyl alcohol and optionally comprising an antihalation dye orpigment as disclosed in EP-A- 410 500.

In another embodiment, the intermediate layer is a layer comprisinghydrophobic polymer beads having an average diameter not lower than 0.2mm and having been prepared by polymerization of at least oneethylenically unsaturated monomer. Preferably, said intermediate layerin dry condition comprises said hydrophobic polymer beads in an amountof up to 80% of its total weight. Further details are disclosed in EP-A-483 415.

In still another embodiment, the intermediate layer is a layercomprising particles of a water insoluble inorganic compound having anumber average size not lower than 0.1 mm. Preferably, said intermediatelayer comprises said water insoluble inorganic compound in an amount ofat least 0.1 g/m2. Further details are disclosed in EP-A- 94 203 779.7.

In still another embodiment, the intermediate layer is a layercomprising particles of an alkali insoluble non-polymeric organiccompound having a melting point of at least 50° C., said particleshaving a number average size between 0.1 mm and 10 mm. Preferably, saidintermediate layer comprises said alkali insoluble non-polymeric organiccompound in an amount of at least 0.1 g/m2. Further details aredisclosed in EP-A- 95 201 713.5.

In still another embodiment, the intermediate layer is a layercomprising particles of an alkali insoluble polymeric organic compoundobtainable by polycondensation, said particles having a number averagesize between 0.02 mm and 10 mm. Preferably, said intermediate layercomprises said alkali insoluble polymeric organic compound obtainable bypolycondensation in an amount of at least 0.1 g/m2. Further details aredisclosed in EP-A- 95 203 052.6.

All said intermediate layers may comprise pigment particles with anumber average diameter between 0.2 and 1 μm such as CaCO3, TiO2, BaSO4,Al2O3, SiO2, ZnO2, etc..

A supplemental intermediate layer, which may be present between saidsilver halide emulsion containing layer and said intermediate layer mayincorporate one or more ingredients such as i.a. antihalation dyes orpigment, developing agents, silver halide solvents, base precursors, andanticorrosion substances.

The imaging element may optionally contains an upper layer (antistresslayer) comprising gelatin and preferably at least 30 mg/m² of a mattingagent with a weight average diameter larger than the mean roughnessdepth Rz of the grained and anodized side of the aluminum support whichis coated with the above mentioned layer.

Said matting agent may be a water-insoluble, alkali soluble organicpolymer or copolymer such as a copolymer of acrylic acid and methylacrylate. More preferably said matting agent is a water insolubleinorganic compound such as silica. Most preferably said matting agent isan alkali-insoluble organic polymer or copolymer such as a polymer orcopolymer of methyl (meth)acrylate. A particularly preferred mattingagent is a copolymer of styrene, methyl methacrylate and maleic acid.

The profile of the grained and anodized aluminum support is measuredwith a mechanical perthometer Mahr Perthen S6P containing as measuringhead RTK 50 (tradenames of Feinpruef Perthen GmbH, Goettingen, Germany)equipped with a diamond stylus with a diameter of 5 mm under a pressureof 1.0 mN after completely stripping away all supplemental layers and/orforeign substances according to techniques well known in the art.

The sampling length Ls which is the reference length for roughnessevaluation and is also the cut-off lc of the profile filter used forseparating the wavelengths belonging to roughness from those belongingto waviness measures 0.25 mm. The evaluation length Lm, being that partof the traversing length Lt which is evaluated contains standard 5consecutive sampling lengths. The traversing length Lt is the overalllength travelled by the tracing system when acquiring the profile. It isthe sum of pre-travel, evaluation length Lm and post travel. Pre-travelis the first part of the traversing length Lt. Post-travel is the lastpart of the traversing length Lt. Pre-travel and post-travel arerequired for phase-corrected filtering. Single roughness depth Zi is thevertical distance of the highest to the deepest profile point of onesampling length. Mean roughness depth Rz is the mean value of the singleroughness depths Zi of consecutive sampling lengths.

    Rz=RzDin=1/n(Z1+Z2+. . . +Zn)

According to the invention the matting agent has a weight averagediameter larger than the mean roughness depth Rz of the grained andanodized side of the aluminum support which is coated with the abovementioned layers, preferably larger than 1.2 times said mean roughnessdepth, more preferably larger than 1.3 said mean roughness depth. Theupper limit of the weight average diameter of said matting agent is notso important but is for practical reasons preferably not more than 15μm, more preferably not more than 10 μm.

The mean roughness depth of a grained and anodized aluminum foilsuitable as a support for an imaging element according to the presentinvention ranges preferably from 2 μm to 6 μm, more preferably from 2.5μm to 5 μm.

According to the invention said antistress layer comprises at least 30mg/m² of a matting agent with a weight average diameter larger than saidmean roughness depth, more preferably between 50 and 500 mg/m² of saidmatting agent, most preferably between 100 and 200 mg/m² of said mattingagent.

According to the invention the antistress layer comprises preferablyunhardened gelatin or gelatin admixture preferably in an amount between0.2 and 2 g/m², more preferably between 0.4 and 1.75 g/m², mostpreferably between 0.6 and 1.25 g/m2.

Preferably at least 50%, more preferably at least 75%, most preferablyat least 90% by weight of said unhardened gelatin or gelatin admixturebelongs to one or more gelatin species whereof a 10% by weight aqueoussolution at 36° C. and pH 6 has a viscosity lower than 35 mPa·s, morepreferably lower than 30 mPa·s at a shearing rate of 1000 s-1.

Said antistress layer can comprise more than one species of unhardenedgelatin whereof a 10% by weight aqueous solution at 36° C. and pH 6 hasa viscosity lower than 35 mPa·s at a shearing rate of 1000 s-1, but itis preferred for practical reasons that said layer comprises only onesuch gelatin.

Said antistress layer may contain small particles e.g. matting agentswith a mean diameter between 0.2 and 3 μm in order to improve thediffusion of processing solutions through said antistress layer.

The antistress layer is the surface layer of the imaging element lyingon that side of the support that carries the emulsion layer.

The antistress layer and the layer containing the silver halide emulsionhave to be in water permeable relationship with said image receivinglayer in the imaging element.

According to the present invention the imaging element can beinformation-wise exposed in an apparatus according to its particularapplication. A wide choice of cameras for exposing the photosensitivesilver halide emulsion exists on the market. Horizontal, vertical anddarkroom type cameras and contact-exposure apparatus are available tosuit any particular class of reprographic work. The imaging element inaccordance with the present invention can also be exposed with the aidof i.a. laser recorders and cathode rays tubes.

According to the present invention the development and diffusiontransfer of the information-wise exposed imaging element in order toform a silver image in said photosensitive layer and to allow unreducedsilver halide or complexes formed thereof to diffuse image-wise from thephotosensitive layer to said image receiving layer to produce therein asilver image, are effected with the aid of an aqueous alkaline solutionin the presence of (a) developing agent(s), and (a) silver halidesolvent(s). The developing agent(s) and/or the silver halide solvent(s)can be incorporated in the aqueous alkaline solution and/or in theimaging element.

Preferably a silver halide solvent in the aqueous alkaline solution isused in an amount between 0.05% by weight and 5% by weight and morepreferably between 0.5% by weight and 2% by weight.

The silver halide solvent, which acts as a complexing agent for silverhalide, preferably is a water-soluble thiosulphate or thiocyanate e.g.sodium, potassium, or ammonium thiosulphate and sodium, potassium, orammonium thiocyanate.

Further silver halide solvents that can be used in connection with thepresent invention are e.g. sulphite, amines, 2-mercaptobenzoic acid,cyclic imide compounds such as e.g. uracil, 5,5-dialkylhydantoins, alkylsulfones and oxazolidones.

Further silver halide solvents for use in connection with the presentinvention are alkanolamines. Examples of alkanolamines that may be usedin connection with the present invention correspond to the followingformula: ##STR1##

wherein X and X' independently represent hydrogen, a hydroxyl group oran amino group, l and m represent 0 or integers of 1 or more and nrepresents an integer of 1 or more. Said alkanolamines may be present inthe alkaline processing liquid in a concentration preferably between0.1% and 5% by weight. However part or all of the alkanolamine can bepresent in one or more layers of the imaging element.

Still other preferred further silver halide solvents for use inconnection with the present invention are thioethers. Preferably usedthioethers correspond to the following general formula:

    Z-(R.sup.1 -S)t-R.sup.2 -S-R.sup.3 -Y

wherein Z and Y each independently represents hydrogen, an alkyl group,an amino group, an ammonium group, a hydroxyl, a sulfo group, acarboxyl, an aminocarbonyl or an aminosulfonyl, R¹, R² and R³ eachindependently represents an alkylene that may be substituted andoptionally contain an oxygen bridge and t represents an integer from 0to 10. Examples of thioether compounds corresponding to the aboveformula are disclosed in e.g. U.S. Pat No. 4,960,683 and EP-A- 554 585.

Still further suitable silver halide solvents are1,2,4-triazolium-3-thiolates, preferably 1,2,4-triazolium-3-thiolatessubstituted with at least one substituent selected from the groupconsisting of a C₁ -C₈ alkyl group that contains at least 3 fluorineatoms, a C₄ -C₁₀ hydrocarbon group and a 4-amino group substituted witha C₁ -C₈ alkyl group that contains at least 3 fluorine atoms and/or a C₄-C₁₀ hydrocarbon group.

Combinations of different silver halide solvents can be used and it isalso possible to incorporate at least one silver halide solvent into asuitable layer of the imaging element and to add at least one othersilver halide solvent to the developing solution.

The alkaline processing liquid may also contain (a) developing agent(s).In this case the alkaline processing liquid is called a developer. Onthe other hand some or all of the developing agent(s) may be present inone or more layers of the photographic material or imaging element. Whenall of the developing agents are contained in the imaging element thealkaline processing liquid is called an activator or activating liquid.

Silver halide developing agents for use in accordance with the presentinvention are preferably of the p-dihydroxybenzene type, e.g.hydroquinone, methylhydroquinone or chlorohydroquinone, preferably incombination with an auxiliary developing agent being a1-phenyl-3-pyrazolidone-type developing agent and/orp-monomethylaminophenol. Particularly useful auxiliary developing agentsare the 1-phenyl-3-pyrazolidones. Even more preferred, particularly whenthey are incorporated into the photographic material are1-phenyl-3-pyrazolidones of which the aqueous solubility is increased bya hydrophilic substituent such as e.g. hydroxy, amino, carboxylic acidgroup, sulphonic acid group etc.. Examples of 1-phenyl-3-pyrazolidonessubstituted with one or more hydrophilic groups are e.g.1-phenyl-4,4-dimethyl-2-hydroxy-3-pyrazolidone,1-(4-carboxyphenyl)-4,4-dimethyl-3-pyrazolidone etc.. However otherdeveloping agents can be used.

Preferred amounts of the hydroquinone-type developing agents are in therange of 0.05 mole to 0.40 mole per liter and preferred amounts ofsecondary developing agent(s) in the range of 1.8×10⁻³ to 2.0×10⁻¹ moleper liter.

The aqueous alkaline solution in accordance with the present inventionmay further comprise sulphite e.g. sodium sulphite in an amount rangingfrom 40 g to 180 g per liter, preferably from 60 to 160 g per liter incombination with another silver halide solvent.

The quantitative ranges given for the developing agents, silver halidesolvents, and sulphite apply to the amount of these compounds present assolutes in the aqueous alkaline solution during the DTR-processing,whether these compounds make part of the aqueous alkaline solution orwere dissolved from the layers containing them upon application theretoof the aqueous alkaline solution.

The aqueous alkaline solution suitable for use according to the presentinvention preferably comprises aluminum ions in an amount of at least0.3 g/l, more preferably in an amount of at least 0.6 g/l in order toprevent sticking of the emulsion layer to the transporting rollers whenthe emulsion is swollen with the aqueous alkaline solution.

The alkaline processing liquid preferably has a pH between 9 and 14 andmore preferably between 10 and 13, but depends on the type of silverhalide emulsion material to be developed, intended development time, andprocessing temperature.

The processing conditions such as temperature and time may vary withinbroad ranges provided the mechanical strength of the materials to beprocessed is not adversely influenced and no decomposition takes place.

The pH of the alkaline processing liquid may be established by anorganic or inorganic alkaline substance or a combination thereof.Suitable inorganic alkaline substances are e.g. hydroxides of sodium andpotassium, alkali metal salts of phosphoric acid and/or silicic acide.g. trisodium phosphate, orthosilicates, metasilicates,hydrodisilicates of sodium or potassium, and sodium carbonate etc..Suitable organic alkaline substances are e.g. alkanolamines. In thelatter case the alkanolamines will provide or help providing the pH andserve as a silver halide complexing agent.

The aqueous alkaline solution may further comprise hydrophobizing agentsfor improving the hydrophobicity of the silver image obtained in theimage receiving layer. Generally these compounds contain a mercaptogroup or thiolate group and one or more hydrophobic substituents.Particularly preferred hydrophobizing agents aremercapto-1,3,4-thiadiazoles as described in DE-A- 1 228 927 and in U.S.Pat. No. 4,563,410, 2-mercapto-5-heptyl-oxa-3,4-diazole and long chain(at least 5 carbon atoms) alkyl substituted mercaptotetrazoles. Thehydrophobizing agents can be used alone or in combination with eachother.

These hydrophobizing compounds can be added to the aqueous alkalinesolution in an amount of preferably 0.1 to 3 g per liter and preferablyin admixture with 1-phenyl-5-mercaptotetrazole, the latter compound maybe used in amounts of e.g. 50 mg to 1.2 g per liter of solution, whichmay contain a minor amount of ethanol to improve the dissolution of saidcompounds.

The aqueous alkaline solution may comprise other ingredients such ase.g. oxidation preservatives, calcium-sequestering compounds,anti-sludge agents, and hardeners including latent hardeners.

Regeneration of the aqueous alkaline solution according to known methodsis, of course, possible, whether the solution incorporates developingagent(s) and/or silver halide solvent(s) or not.

The development may be stopped--though this is often not necessary--witha so-called stabilization liquid, which actually is an acidic stop-bathhaving a pH preferably in the range from 5 to 7.

Bufferred stop bath compositions comprising a mixture of sodiumdihydrogen orthophosphate and disodium hydrogen orthophosphate andhaving a pH in said range are preferred.

The development and diffusion transfer can be initiated in differentways e.g. by rubbing with a roller, by wiping with an absorbent meanse.g. with a plug of cotton or sponge, or by dipping the material to betreated in the liquid composition. Preferably, they proceed in anautomatically operated apparatus. They are normally carried out at atemperature in the range of 18° C. to 30° C. and in a time from 5 s to 5min.

After formation of the silver image in the image receiving layer anexcess of aqueous alkaline solution still present on the base may beeliminated, preferably by guiding the foil through a pair of squeezingrollers.

The silver image thus obtained in the layer of physical developmentnuclei is subsequently uncovered by treating the imaging element toremove all the layers above the layer containing physical developmentnuclei, thereby exposing the imaged surface of the aluminum support.

According to a particularly preferred embodiment of the presentinvention the silver image in the layer of physical development nucleiis uncovered by washing off all the layers above the layer containingphysical development nuclei with rinsing water.

The temperature of the rinsing water may be varied widely but ispreferably between 30° C. and 50° C., more preferably between 35° C. and45° C.

The imaged surface of the hydrophilic surface of a support can besubjected to a chemical treatment that increases the hydrophilicity ofthe non-silver image parts and the oleophilicity of the silver image

This chemical after-treatment is preferably carried out with alithographic composition often called finisher comprising at least onecompound enhancing the ink-receptivity and/or lacquer-receptivity of thesilver image and at least one compound that improves the ink-repellingcharacteristics of the hydrophilic surface.

Suitable ingredients for the finisher are e.g. organic compoundscontaining a mercapto group such as the hydrophobizing compoundsreferred to hereinbefore for the alkaline solution. Preferred compoundscorrespond to one of the following formulas: ##STR2##

wherein R⁵ represents hydrogen or an acyl group, R⁴ represents alkyl,aryl or aralkyl. Most preferably used compounds are compounds accordingto one of the above formulas wherein R⁴ represents an alkyl containing 3to 16 C-atoms. Said (a) hydrophobizing agent(s) is(are) comprised in thefinisher preferably in a total concentration between 0.1 g/l and 10 g/l,more preferably in a total concentration between 0.3 g/l and 3 g/l.

Additives improving the oleophilic ink-repellency of the hydrophilicsurface areas are e.g. carbohydrates such as acidic polysaccharides likegum arabic, carboxymethylcellulose, sodium alginate, propylene glycolester of alginic acid, hydroxyethyl starch, dextrin,hydroxyethylcellulose, polyvinyl pyrrolidone, polystyrene sulphonicacid, polyglycols being the reaction products of ethyleneoxide and/orpropyleneoxide with water or an alcohol and polyvinyl alcohol.Optionally, hygroscopic substances e.g. sorbitol, glycerol,tri(hydroxyethyl)ester of glycerol, and turkish red oil may be added.

Furthermore (a) surface-active compound(s) is preferably also added tothe finisher. The concentration thereof may vary within broad rangesprovided the finisher shows no excessive degree of foaming when platesare finished. Preferred surface-active compound are anionic or non-ionicsurface-active compound.

A suitable finisher as disclosed in U.S. Pat. No. 4,563,410 is acomposition comprising a solution of a mercaptotriazole in a solution ofpolyethylene oxide with a molecular weight of 4,000. Further suitablefinishers have been described in i.a. U.S. Pat. No. 4,062,682 and EP-A-681 219.

At the moment the treatment with the finisher is started the surfacecarrying the silver pattern may be in dry or wet state. In general, thetreatment with the finisher does not take long, usually not longer thanabout 30 seconds and it may be carried out immediately after theprocessing and uncovering steps, preferably at a temperature of thefinisher in the range from 30° C. to 60° C.

The finisher can be applied in different ways such as by rubbing with aroller, by wiping with an absorbent means e.g. with a plug of cotton orsponge, or by dipping the material to be treated in the finisher. Theimage-hydrophobizing step of the printing plate may also proceedautomatically by conducting the printing plate through a device having anarrow channel filled with the finisher and conveying the printing plateat the end of the channel between two squeezing rollers removing theexcess of liquid.

As soon as the hydrophilic surface of a support carrying the silverimage has been treated with the finisher, it is ready to be used as aprinting plate.

The following example illustrates the present invention without however,limiting it thereto. All parts, percentages and ratios are by weightunless otherwise indicated.

EXAMPLE 1

A 0.30 mm thick aluminium foil (AA 1050) was degreased by immersing thefoil in an aqueous solution containing 10% phosphoric acid andsubsequently etched in an aqueous solution containing 2 g/l of sodiumhydroxide. The foil was then electrochemically grained using analternating current in an aqueous solution containing 4 g/l ofhydrochloric acid and 4 g/l of hydroboric acid at a temperature of 35°C. to form a surface topography with a mean roughness depth of 4 μm. Thealuminium plate was then desmutted with an aqueous solution containing30% of sulfuric acid at 60° C. for 120 seconds. The foil wassubsequently subjected to anodic oxidation in a 20% sulfuric acidaqueous solution to form an anodic oxidation film on the front side of3.0 g/m² of Al₂ O₃.H₂ O, treated with an aqueous solution containing 20g/l of NaHCO₃ at 45° C. for 30 sec and then rinsed with demineralisedwater and dried.

The imaging element I was obtained by coating the grained, anodized andposttreated aluminum support with a silver-receptive stratum containing4 mg/m² Ag° as physical development nuclei.

An intermediate layer was then provided on the dry silver-receptivestratum from an aqueous composition of polyvinyl alcohol and Levanyl Rot(trade name of Bayer A.G. for a red pigment) while the grained andanodized aluminum support is in contact with a guiding roller at 22° C.in such a way that the resulting dried layer had a weight of 120 mg ofpolyvinyl alcohol and 250 mg of Levanyl Rot per m².

At the same time a substantially unhardened photosensitivenegative-working cadmium-free orthochromatically sensitized gelatinsilver chloroiodide emulsion layer (99.8/0.2 mol%) containing 1mmole/mole AgX of 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene and 2.2mole/ mole AgX of 1-(3-(2-sulphobenzamido))phenyl-5-mercapto-tetrazolewas coated on the intermediate layer, the silver halide being providedin an amount corresponding to 2.50 g of silver nitrate per m² and thegelatin content of the emulsion layer being 1.50 g/m², consisting of 0.7g/m² of a gelatin with a viscosity of 21 mPas and the remainder of agelatin with a viscosity of 14 mPas.

Finally the photosensitive emulsion layer was overcoated with anantistress layer containing no hardeners comprising 0.7 g/m² gelatinwith a viscosity of 10-12 mPas (gelatin K 7598 of Koepff), 60 mg/m2 ofLevanyl Rot and 140 mg/m² of a copolymer of styrene, methyl methacrylateand maleic acid as matting agent with a weight average diameter of 3.5μm.

Imaging element II was obtained in an identical way as imaging element Iwith the exception that the grained, anodized and posttreated aluminumfoil was in contact with a guiding roller at 50° C. when the coatingwith the intermediate layer and the photosensitive layer was carriedout.

Said imaging elements were placed in contact with a test target andexposed therethrough in a process-camera. In the next step each samplewas immersed for 10 s at 24° C. in a freshly made developing solutionhaving the following composition:

    ______________________________________                                        carboxymethylcellulose    4      g                                              sodium hydroxide 22.5 g                                                       anhydrous sodium sulphite 120 g                                               hydroquinone 20 g                                                             1-phenyl-4-methyl-3-pyrazolidinone 6 g                                        potassium bromide 0.75 g                                                      anhydrous sodium thiosulphate 8 g                                             ethylene diamine tetraacetic acid tetrasodium salt 2 g                        demineralized water to make 1000 ml                                           pH (24° C.) = 13                                                     ______________________________________                                    

The initiated diffusion transfer was allowed to continue for 20 s toform a silver image in the image receiving layer.

To remove the developed silver halide emulsion layer, the intermediatelayer and the anti-stress layer from the aluminium foil the developedmonosheet DTR materials were rinsed for 5 s with a water jet at 40° C.in a LP 82 (tradename of a processor marketed by Agfa-Gevaert, Belgium).

Next, the imaged surfaces of the aluminium foils were treated in a LP 82(marketed by Agfa-Gevaert, N.V. of Belgium) for 15 s with a finisher at45° C. to enhance the water-receptivity of the non-image areas and tomake the image areas oleophilic ink-receptive. The finisher had thefollowing composition

    ______________________________________                                        Gebo (trade mark for a surfactant sold by                                                               250    mg                                             Chemische Fabrik Chem-Y, Gmbh, Germany)                                       polyethylene glycol 3000 75 g                                                 potassium nitrate 12.5 g                                                      citric acid 20.0 g                                                            2-mercapto-5-heptyl-oxa-3,4-diazole 2.0 g                                     NaH.sub.2 PO.sub.4.2H.sub.2 O 20.0 g                                          5-bromo-5-nitro-1,3-dioxane 200 mg                                            sodium hydroxyde 13.0 g                                                       water to make 1000 ml                                                         pH (20° C.) = 5.9                                                    ______________________________________                                    

The printing plates thus prepared were used for printing under identicalconditions. The printing plates were mounted on the same offset printingmachine (HEIDELBERG GTO-46). As fountain solution was used Rotamatic at50% and as ink K+E 123W from Kast and Ehinger, A. G., Germany. Acompressible rubber blanket was used.

The quality of the printing plates was judged by the number and vastnessof the white spots in the image areas and the number and vastness of thehorseshoes in the non-image areas. The results are given in table 1. Alower number for said items yields of course a better plate.

                  TABLE 1                                                         ______________________________________                                        white spots          horseshoes                                               element diameter  air bubble number                                                                              diameter                                   ______________________________________                                        I       0.2-0.35 mm                                                                             0.25 mm    5.5/m.sup.2                                                                         0.5-0.7 mm                                   II 0.20 mm 0.15 mm 2.5/m.sup.2 <0.3 mm                                      ______________________________________                                    

It is clear from the results in table 1 that the imaging element, thatis prepared by coating the emulsion layer while the support is on aheated guiding roller yields a much better plate than an printingelement that is prepared by coating the emulsion layer while the supportis on a guiding roller at room temperature.

What is claimed is:
 1. A method for preparing an imaging elementcomprising the steps of coating in the order given on a grained andanodized side of an aluminum support (i) an image receiving layercontaining physical development nuclei, (ii) a photosensitive layercontaining a silver halide emulsion being in water permeablerelationship with said image receiving layer and (iii) optionally anantistress layer being in water permeable relationship with said imagereceiving layer comprising unhardened gelatin, characterized in thatsaid aluminum support when being coated with said photosensitive layeris in contact with a guiding roller, heated at a temperature between 35and 70° C.
 2. A method for preparing an imaging element according toclaim 1 wherein said guiding roller is a bare guiding roller.
 3. Amethod for preparing an imaging element according to claim 2 whereinsaid bare guiding roller is a bare metallic guiding roller.
 4. A methodfor preparing an imaging element according to claim 1 wherein saidguiding roller is heated at a temperature between 40 and 60° C.
 5. Amethod for preparing a lithographic plate comprising the steps of:(i)image-wise exposing an imaging element according to claim 1; (ii)applying an aqueous alkaline solution to the imaging element in thepresence of at least one developing agent and at least one silver halidesolvent; (iii) treating the imaging element to remove the layer(s) ontop of said image receiving layer, thereby uncovering said silver imageformed in said image receiving layer.